1. Field of the Invention
The invention relates to a cotyloid cavity prosthesis whose bearing insert is held in an outer shell by means of a locking device which comprises an undercut surface on the outer shell and, engaging behind it, an elastic lip projecting obliquely outwards from the bearing insert towards the open side of the prosthesis.
2. Background of the Invention
In known cotyloid cavity prostheses of this type, the bearing insert made of polyethylene is provided with an elastic retention lip which snaps in behind projections of the outer shell (U.S. Pat. No. 5,133,764). To ensure that the bearing insert sits firmly in the outer shell, the aim is to avoid any play between the lip and the undercut surface. However, since manufacturing tolerances have to be taken into consideration, this aim cannot be achieved in practice.
The invention is a cotyloid cavity prosthesis of the type which allows the insert to be mounted in the shell in a manner free from play, but with tolerance compensation.
According to the invention, once the bearing insert is mounted in the outer shell, the lip of the bearing insert rests with its tip on the undercut surface, and the undercut surface forms with the mounting device, at the contact point between the tip of the lip and the undercut surface, a smaller angle than the tangent on the deformation line of the tip of the lip at the contact point with the undercut surface. The lip of the bearing insert rests with its tip in a self-locking manner on the undercut surface, with bending deformation of the tip.
The deformation line is the curve on which the tip of the lip, upon bending deformation of the lip, moves under the force exerted by the undercut surface on its tip. At the contact point, this curve has the direction in which the tip of the lip held securely by the undercut surface would move further outwards if the undercut surface were not present. The reference direction for the direction of the undercut surface and of the deformation line of the tip of the lip is the direction of mounting of the insert. It is immaterial which reference direction is chosen for the comparison of these two directions. For the sake of simplicity, the center axis of the prosthesis is specified in this application, but it is not decisive how the direction of this center axis is determined in complex prosthesis shapes, since its exact direction is not the issue, as long as it is fixed.
Under the force of the bending deformation of the lip, the tip of the lip exerts in the radial direction, and in particular in the axial direction, an elastic force on the undercut surface, whose reactive force presses the bearing insert further into the outer shell, until it reaches an end position, free from play, on a limit stop member which is formed, for example, by the bottom of the cotyloid cavity. In doing so, the lip preferably interacts with the undercut surface in a self-locking manner. This is to be understood as meaning that the geometric relationships of the interaction of lip and undercut surface are chosen such that, under the effect of a force seeking to release the insert from the outer shell, the lip cannot slide out along the undercut surface once the lip and the undercut surface are elastically pushed together and cannot snap out, but is instead held securely by friction.
To ensure that this state of assembly is obtained in all cases, the dimensions of the undercut surface and of the lip are chosen in such a way that, even with the greatest deviations in tolerance, the lip in all cases finds the undercut surface. This condition is easy to fulfill since the undercut surface can be designed with an axial extent which is greater than all dimensional tolerances arising in this direction. The invention further provides that all areas of the undercut surface which are intended for contact with the tip of the lip have a diameter which is smaller than the unstressed diameter of the tip of the lip.
In known prostheses of similar type, the lip extends away inwards from the open side of the prosthesis (U.S. Pat No. 5,507,828) and presses radially against the outer shell; the lip does not interact with the contact surface in a self-locking manner and can slide outwards and snap out of place under the effect of a force acting axially towards the open side of the prosthesis.
The undercut surface with which the lip of the bearing insert interacts can be part of a projection, for example of a bead, or of a recess, for example a groove, in particular an annular groove. The recess or the projection can be provided along the whole circumference of the bearing insert or only in parts thereof, and the lip can accordingly be designed as an annular lip or consist of several segments which interact, for example like fingers, with projections designed as catches the above description, it has been assumed that the lip is arranged on the bearing insert and the undercut surface is arranged on the cotyloid cavity, but it is also possible, the other way round, for the lip to be provided on the cotyloid cavity and for the undercut surface to be provided on the bearing insert if the material properties so permit.